Roll bars for motor vehicles are usually used to elastically couple the wheel suspension of a wheel on one side of the motor vehicle with the wheel suspension of the corresponding wheel of the same axle on the other side of the motor vehicle. The coupling is performed such that when one wheel is deflected inwardly, the spring action of the other wheel is also acted on in the direction of inward deflection. As a result, the lateral slope of the motor vehicle toward the outside of the curve is reduced during the travel of the motor vehicle in a curve, partly because the wheel suspension of the particular wheel that is the outer wheel in the curve is additionally supported by the spring system of the wheel suspension of the wheel that is the inner wheel in the curve and partly because the wheel suspension of the wheel that is the inner wheel in the curve is forced somewhat in the direction of inward deflection relative to the chassis of the vehicle.
By contrast, the roll bar shall not possibly affect the spring action characteristics of the vehicle during straight-line travel.
However, if the pavement is so uneven that one wheel on one side of the vehicle is forced in the inward deflection direction, while the corresponding wheel on the other side of the vehicle must be moved in the outward deflection direction to maintain the desired road contact, the driving smoothness is compromised by a roll bar, because the roll bar tends to counteract mutually opposite movements of the wheel suspensions coupled by the roll bar relative to the vehicle body. Thus, during straight-line travel, a roll bar may undesirably cause vibrations of one wheel to be transmitted to the opposite wheel of the same axle, which compromises the driving smoothness.
This contradiction between the safety and comfort requirements imposed on a roll bar can be eliminated if the roll bar is switched off during straight-line travel and is again switched on automatically during travel in a curve.
Such a system has been known from, e.g., DE-AS 11 05 290. It is described in DE-AS 11 05 290 that a stabilizer with split torsion bar is provided, the sections being connected to one another in the manner of a hydraulic clutch, which are controlled as a function of the centrifugal force or the steering of the vehicle. Thus, it is possible to inactivate the stabilizer during straight-line travel by releasing the clutch and to switch it on during travel in a curve. It is also known from this document that the sections of the torsion bar can be adjusted in relation to one another during travel in a curve by means of a motor operator such that the vehicle body slopes less far toward the outside of the curve, i.e., a sloping is actively counteracted.
A prior-art actuator for coupling roll bars with split torsion bar is known from DE 199 50 244 C2. Other actuators are described in DE 37 40 24 C2.
The drawback of the prior-art roll bars with torsion bar is that the switching on (coupling) of the roll bar during travel in a curve must take place, in general, automatically and very rapidly for safety reasons, because the vehicle could otherwise become uncontrollable in the curve. The high costs of the prior-art roll bars which are associated with these requirements on the actuator have caused that switch-on roll bars (also known under the name “active roll bars”) are not used in models manufactured in large series (i.e., in vehicles manufactured in large numbers).
Furthermore, slow, manually switchable roll bars are known, which are used especially in off-the-road vehicles. However, such roll bars, known, e.g., from DE 43 07 639 C1, are suitable only for improving the traction during slow travel on the terrain. Such systems are not suitable for safety reasons for switching on and off the roll bar during travel on a normal road, because there is a risk that the roll bar will not be switched on or will not be switched on in time in a curve, and the vehicle will thus assume an uncontrollable state.